1. Field of the Invention
This invention relates to an optical recording medium which is capable of performing information recording and reproduction by irradiation of light beam.
2. Description of the Prior Art
As the optical recording medium such as the so-called "optical disc", etc., there have heretofore been known those which are made up of a thin film of alloy of rare earth metals and transition metals; a thin film of reductive oxides such as chalcogen compounds, etc. utilizing a phase transition from amorphous state to crystalline state; heat-mode recording materials; thermoplastic recording materials; and so on, all being used in the form of an optical recording layer. For instance, those which are called the magneto-optical recording medium are made up of polycrystalline thin films such as MnBi, MnCuBi, etc.; amorphous thin films such as GdCo, GdFe, TbFe, DyFe, GdTbFe, TbDyFe, etc.; and single crystal thin films such as GIG (gadolinium-iron-garnet), etc.
Of these thin films, the amorphous thin films are nowadays considered excellent as the optical recording medium taking into account the film forming property at the time of fabricating a thin film of a large area at a temperature in the neighborhood of a room temperature, the write-in efficiency for writing information signals with a small energy of light, and the read-out efficiency for reading the signals as written in with a good S/N ratio. For instance, GdTbFe has a large Kerr rotational angle and a Curie point of 150.degree. C. or so, hence it is suitable for use as the magneto-optical recording medium. As the result of further studies and researches for improving the Kerr rotational angle, it has been found that GdTbFeCo is able to provide the magneto-optical recording medium having a sufficiently large Kerr rotational angle and being capable of performing the read-out operation with good S/N ratio.
However, GdTbFe, GdTbFeCo, and other amorphous magnetic materials have various disadvantages such that they are inferior in the corrosion-resistant property, and are oxidized in an atmosphere containing moisture to bring about deterioration in their magnetic characteristics. The deterioration in the characteristics of the optical recording layer owing to such corrosion has been the common problem with the magneto-optical recording medium as well as the abovementioned optical recording medium.
With a view to solving such disadvantage as mentioned above, there have so far been proposed the optical recording medium of various constructions such as one provided with a protective layer on the optical recording layer, or an air-sandwich structure with an inert gas being sealed in the interior of the recording medium, or a stick-together structure with a protective substrate being adhered onto the optical recording layer. However, the cause for corrosion of the optical recording medium is thought to be due to staining of the substrate, or adhesion of oil and dirt to, or intrusion of moisture and oxygen into, the recording medium during its handling, etc. after the fabrication, and, even with such structure as mentioned above, the corrosion is apt to take place from the end surface of the substrate and to spread inward. On account of this, it has heretofore been the practice to prevent defects in the recorded information and/or extinction of the accumulated information by not using, as the recording part, a portion of the whole optical recording layer usually formed on the entire surface of the substrate, which corresponds to the edge of the substrate. However, once the corrosion starts in one part of the optical recording medium, a local battery is formed in that place, and the corrosion is accelerated around that place, which ultimately proceeds to the recording portion. As the consequence of this, the aforedescribed methods did not give fundamental solution to the corrosion of the optical recording layer.